This invention is directed to graphite-containing lubricating compositions. More particularly, this invention is directed to energy efficient graphite-containing gear oil lubricating compositions.
Examples of lubricating compositions containing graphite are disclosed in the following U.S. patents. In U.S. Pat. No. 4,094,799 (June 13, 1978) of D. L. DeVries and J. M. DeJovine, an improved lubricating composition is disclosed which overcomes to a superior degree the tendency to form varnish, sludge and other similar deposits in the presence of solid lubricants dispersed in such compositions. The improvement was the result of employing at least one copolymer of (1) an N-vinyl pyrrolidone, and (2) an oil soluble acrylic ester. In view of intended end use in a modern internal combustion engine, other additives which may be included to impart particular properties to a lubricating composition were disclosed such as detergents, antioxidants, metal deactivators, pour point depressants, oiliness agents, blooming agents, and peptizing agents.
In U.S. Pat. No. 4,132,656 (Jan. 2, 1979) of D. L. DeVries and J. M. DeJovine, an improved lubricating oil composition provided to a surprising degree a reduction in detrimental deposit formations, e.g., on internal combustion engine components lubricated by such composition of the disclosed invention. The composition included certain nitrogen-containing polymers. The nitrogen-containing polymers consisted of a graft of dialkyl-aminoalkylmathacrylate to an oil soluble, substantially linear terpolymer. The terpolymer consisted of the reaction product of ethylene, a terminally unsaturated straight-chain alkene of 3 to about 12 carbons, and a terminally unsaturated nonconjugated alkadiene of 5 to about 8 carbons. The terpolymer had an average carbon chain length of about 700 to about 7,000 and an inherent viscosity of about 0.6 to about 1.9 at 0.10 weight percent solution in tetrachloroethylene at 30.degree. C. Methods for preparing these dispersing agents are disclosed.
In U.S. Pat. No. 4,134,844 (Jan. 16, 1979) of D. L. DeVries and J. M. DeJovine, there is disclosed an improved lubricating composition which provides improved inhibition of sludge and varnish during use. The composition contained solid particles, preferably of graphite, along with other commonly used oil additives. Examples of such additives are conventional antioxidants, metal deactivators, pour point depressants, oiliness agents, blooming agents, and peptizing agents. A critical dispersing agent is added, which consists of the reaction product of an oxidized copolymer of ethylene and propylene with a formaldehyde-yielding reactant, and an aliphatic amine of polyamine. Suitable ethylene-propylene copolymers which are to be oxidized consist of disclosed reaction products of propylene, ethylene, aliphatic or cycloaliphatic radicals having a terminal double bond of from 2 to about 20 carbon atoms and a diolefin containing from 4 to about 25 carbon atoms. The diolefins may be either conjugated or nonconjugated diolefins. Methods for preparing the particular polymers of the critical dispersing agent are disclosed.
In U.S. Pat. No. 4,136,040 (Jan. 23, 1979) of D. L. DeVries and J. M. DeJovine, there is disclosed an improved lubricating composition which employs solid lubricants dispersed with the aid of a nitrogen-containing mixed ester of a carboxyl-containing interpolymer. The carboxyl-containing interpolymers include interpolymers of alpha, beta-unsaturated acids, or anhydrides. The reaction product of the carboxyl-containing interpolymer is then esterified, followed by a neutralization of the unesterified pendent carboxy acid groups with a polyamine. Preferred polyamino compounds include primary-aminoalkyl substituted tertiary amines, hetercyclic amines, and the like. The amino compounds may be aeromatic or alaphatic amines and are preferably heterocyclic amines such as aminoalkyl substituted morphylenes, piperyzenes, pyridines, and the like. The amines have from 4 to about 30 carbon atoms, preferably from about 4 to about 12 carbon atoms. Polar substituents such as halides of chlorine, bromine, ether, nitro, nitryl, and the like may be present.
Unfortunately, preparation of graphite-containing gear oils using the same graphite-containing concentrates useful in the preparation of lubricating oil compositions discussed hereinabove did not work particularly well. It was discovered that, due to differences in additive packages required for the use conditions of a gear oil as compared to that of an automotive crank case lubricating oil, there resulted a thickening of the gear oil in use. Example 1 of this application demonstrates that thickening occurred when a gear oil made with the same graphite-containing concentrates useful in the preparation of motor oil compositions discussed hereinabove was used in an automotive differential after only a few thousand miles. Also discussed in Example 1 is a process of agglomeration evidenced by a clumping or lumping together of graphite particles. As extreme pressure (EP) additive packages and dispersant additives both interact, both thickening and agglomeration occur in increasing amounts. With agglomeration, the more uniform film forming properties of dispersed graphite particles diminish. Decreasing film forming properties result in a lessening of the effectiveness of graphite for lubrication. Thickening due to oxidation of the base oil, however, can occur independently of any agglomeration.
By a gear oil composition in this specification is meant a composition suitable for lubricating gears that perform one or more of the following four functions: changing speed, transmitting power, changing rotational direction, and/or changing points of power transmitted. Gear lubricants are herein identified as falling within classification SAE J306C.
The requirements for a gear oil lubricating composition arise out of rolling and sliding motions that are present as gears interact and transmit power and changes in direction. These motions of rolling and sliding occur simultaneously and independently. To prevent failures, gear oil lubricants must provide a hydrodynamic wedge in low to medium contact pressures, e.g., less than about 30,000 pounds per square inch (psi), but require EP additives above 30,000 psi as the gear rotates in the presence of the lubricant. This wedge and EP film layer separate the teeth as the gears mesh. Increasing loads cause pressure contact zones of the gear lubricant to increase, thereby causing the EP film formed to be reduced in thickness. Failure results when both the hydrodynamic wedge and EP film fail to prevent contact between the meshing surfaces of the gears, which eventually results in wear.
To assure proper lubrication and extended service life, the following gear lubricant properties generally are required by substantially all gear lubricants. These properties are analyzed and determined by reference to standard ASTM tests. These properties are gravity, viscosity, viscosity index, flash point, chemical and thermal oxidative stability, anticorrosion tendencies, demulsibility, antifoaming tendencies, extreme pressure, pour point, and acid number. Gravity is the relative density which expresses the weight of a measured volume of product. Viscosity is the flow characteristic of a lubricant at a given temperature. Viscosity index is the empirical number indicating the effects of temperature changes on viscosity. Flash point is the lubricant's temperature, which gives off sufficient vapor to form a flammable mixture with air. Chemical and thermal oxidative stability is the amount of resistance of the lubricant to break down under heat in the presence of oxygen, thereby forming oxygen-containing products. Anticorrosion tendencies is the ability of a lubricant to protect against corrosion such as rust and some other forms of wear such as are observed when there are pits in tooth surfaces as a result of dissolving away portions of the gear. Demulsibility is the ability of a lubricant to separate from water under a given temperature. Antifoaming tendencies is the ability of a lubricant to avoid formation of foams. The tendency to foam is related to surface tension of the composition. Extreme pressure (EP) is a load bearing quality of the lubricant. Pour point is a low temperature property of the lubricant at which temperature the product congeals and gravity flow stops. Acid numbers measure the acidity of the lubricant.
The main difference between a lubricating oil composition of the above-cited patents suitable for use as an automobile lubricating crank case oil composition and a gear oil is the requirement placed upon the gear oil by the following: (1) very high pressures per unit area; (2) the presence of roll/slide contacting processes unique to meshing gears; and (3) the relatively longer use life of gear oils over simple lubricating oils.
The presence of an EP additive in a graphited gear oil has been found to result more readily in formation of either a sludge or agglomeration of the solid lubricating particles than in an automotive lubricating crank case oil. We have discovered a correlation between the amount of EP package breakdown and both agglomeration and thickening. This correlation is discussed and disclosed in Example 2.
Accordingly, there is a need to find stabilizers to be added to graphite-containing gear oil compositions that will inhibit the phenomena of thickening and/or agglomeration discussed hereinabove and in Example 1.